Current limiting fuse

ABSTRACT

Low thermal mass fusible conducting elements are in nucleate boiling relationship to a dielectric liquid for normal large operating currents with resultant high heat transfer coefficient. For currents of short duration and substantially larger in magnitude than normal currents, the fusible conductive elements are in vapor film boiling relationship to the dielectric liquid with resultant heat transfer coefficient which is several orders of magnitude lower than the heat transfer coefficient for nucleate boiling. Accordingly, for currents of very large amplitude and short durations heat buildup and temperature rise of the fusible elements to the melting point thereof is rapid with resultant rapid circuit interruption.

[451 Jan. 9, 1973 Primary Examiner-Bernard A. Gilheany Assistant Examiner-4*". E. Bell I Attorney-Paul A. Frank, John F. Ahern, Julius J. Zaskalicky, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [57] ABSTRACT Low thermal mass fusible conducting elements are in nucleate boiling relationship to a dielectric liquid for normal large operating currents with resultant high heat transfer coefficient. For currents of short duration and substantially larger in magnitude than normal currents, the fusible conductive elements are in vapor film boiling relationship to the dielectric liquid with resultant heat transfer coefficient which is several orders of magnitude lower than the heat'transfer coefficient for nucleate boiling. Accordingly, for currents of Staub et al.

[54] CURRENT LIMITING FUSE [75] Inventors: Fred W. Staub; James C. Corman, both of Scotia; Gunner E. Walmet, Schenectady, all of N.Y. [73] Assignee: General Electric Co.

[22] Filed: June 1,1971 [21] Appl. No.: 148,844

[52] US. Cl....................................337/l66, 337/204 [51] Int. 85/04 [58] Field or Search......337/163, 166., 204, 277, 293,

[56] References Cited a UNITED STATES PATENTS United States Patent I [1 1 d I ii! i .mmm w 1 m st a m =1 2 M ii: d ma 0 .m 9 b Iii!!! PM m m h 1 mmmn c m mm e mmm 2 .1 m msm M mm a: M mmo wamc 1- XX7 6 ii i 13. l 1 i N w 2: 66 W EE MM m w .1 mg m 1.11 l :3 m T m iiiii 1 1 m m m I I m m m m IE. f m m U m 5:: 3 :2 I m m P m 2 I H M 11/ m mmm .l via m R 1% a n 0 B u s t.. 1/41 X m T w 1 1 OiC N f P CAM E G \8* T 59 A H N %%9 m w & WWZ o W I E mm B 599 R 4 man m M 4 9 7 3 (7/ 957 2 322 THERMAL POWER INPUT (WATTS PER SQUARE CENTIMETER) PATENTEUJAN 9191s I CURRENT TRANSITION POINT 32 stun-Skunk 2 I 31 q 30 1 I I F|LM BOILING IO I LC{\NUCLEATE BOIILING I I I INTERFACE EVAPORIATION I0 TEMPERATURE DIFFERENCE (DEGREES CENTIGRADE) If i9. 4 A

INVENTORS FREDW. STAUB JAMES C. CORMAN GUNNAR E. ALMET THE oamsv The present invention relates in general to fuses and more particularly to fuses which will carry high normal currents and yet will provide rapid circuit interruption for undesirably high currents of short duration.

Such large current, high speed fuses are presently available for the protection of devices such as high current carrying semiconductor control devices. Such prior art fuses include fusible elements such as small wires or ribbon, enclosed in a housing containing a gas. The fusible elements are usually embedded in a solidgranular material such as sand or ceramic. While such fuses will melt and interrupt a circuit when large currents of short duration, for example several milliseconds, are passed therethrough, at lower currents of longer time duration excessive element temperature cycling usually results in failure at less than the design failure value.

The present invention is directed to the provision of a current limiting fuse of the kind described above which avoids the disadvantages of the kind describe above in prior art devices.

It is an object of the present invention to provide a fuse with high normal current capacity yet which provides rapid circuit interruption for undesirably high currents of short time duration.

It is another object of the present invention to provide a current limiting fuse of large current carrying capacity yet which is more rapid in response to overload currents.

It is a further object of the present invention to provide a fuse of smaller size than conventional'fuses for a given current rating.

In carrying out the invention as applied to an illustrative embodiment thereof, a sealed enclosure is provided. A pair of electrodes are supported in insulated relationship in the enclosure. A plurality of conductive fusible elements of low thermal mass are located within the enclosure, each connected between the electrodes.

A dielectric liquid partially fills the enclosure and completely submerges the elements. A first predetermined current flow in the elements produces a first predetermined thermal power flow into the liquid which is sufficient to produce nucleate boiling in the liquid. A second greater predetermined current flow in the elements produces a second predetermined thermal power flow into the liquid which is sufficient to produce vapor film boiling in the liquid. As the heat transfer coefficient for the nucleate boiling regime is substantially greater than the heat transfer coefficient for the vapor film boiling regime, thermal energy accumulates in the elements at a rapid rate and raises the temperature thereof at a rapid rate. The elements are constituted of a material and proportioned so that the second predetermined current raises the temperature thereof above the melting temperature of the elements. In view of the low thermal mass of the elements, such temperature rise is rapid. Accordingly, the elements melt and rapidly interrupt current of the second predetermined value flowing through the fuse.

The novel features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, both as to its organization and method of operation, together with v further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

FIG. 1 is a sectional view of a fuse device embodying the present invention.

FIG. 2 is a view of the device of FIG. 1 taken along section lines 2-2 thereof.

FIG. 3 is a diagram of current vs. time showing current pulses of varying amplitude and time duration exchange relationships or regimes existing between the liquid and the heat source for various temperature differences therebetween. This diagram will be useful in explaining the operation of the invention.

FIG. 5 shows a plan view in section of another embodiment of the present invention. 4

FIG. 6 is a side view of the device of FIG. 5.

FIG. 7 is s sectional view of the device of FIGS. 5 and 6 taken along section lines 7--7 of FIG. 6.

FIG. 8 is a sectional view of another embodiment of the present invention showing additional features thereof.

FIG. 9 is an end sectional view of the device of FIG. 8 taken along section lines 9'9 thereof.

Reference is now made to FIGS. 1 and 2 wherein is shown a currentlimiting fuse 10 comprising a housing 11 having a tubular side wall 12 and a pair of electrically non-conducting end walls 13 and 14 sealed thereto to form a sealed enclosure. The cylindrical side wall 12 may be constituted of the material such as copper or aluminum and the end walls 13 and 14 may be constituted of a material such as ceramic suitably bonded to the metal side wall 12. A plurality of fins 15 may be conductively secured to the side wall for increasing the heat transfer coefficient between the side wall 12 and the ambient atmosphere in which the device would be used. A pair of lead-in conductors or electrodes 16 and 17 are provided, extending through respective end walls 13 and 14 into the interior of the .housing. The electrodes 16 and 17 may be made of a material such as copper which is suitably bonded or sealed to the end walls and provides the electrodes or terminals for connecting the fuse in the circuit to be protected. A fusible assembly 20 including a plurality of conductive fusible elements 21 of low thermal capacity or mass and preferably identical are located within the housing. Each of the elements is shown as a wire of circular cross section which may be constituted of a fuse material such as tin, copper, or silver. The elements may also be rectangular or other cross sections. Each of the elements 21 is connected between the electrodes 16 and 17 and suitably bonded thereto, for example, by soldering or cold welding to provide good electrical contact between the elements 21 and the electrodes 16 and 17. The housing or enclosure 11 is partially filled with a dielectric liquid 22 which completely submerges the fusible elements 21 in the operating position of the assembly. The remaining portion 23 of the enclosure is preferably evacuated so that such space is filled with vapors from the dielectric liquid. Of course, the liquid 22 would not solidify at the ambient temperatures in which the fuse would be operated. Also preferably, sufficient liquid is provided in the housing to completely submerge the device regardless of the position thereof. Also preferably, the liquid has arc quenching propertiesto decrease the time of response of the fuse. Each of the fusible elements 21 has a predetermined electrical resistance and v a predetermined surface area. Accordingly, for a predetermined current flow through an element the.

temperature thereof would rise to a predetermined value and a predetermined thermal power flow per unit area or flux would pass through the surface thereof into the dielectric liquid. The aggregate thermal power flow from the elements into the liquid from the aggregate current flow in the elements increases withincreasing current flow and causes an increase in the temperature of the elements.

For the protection of certain electrical devices, such as silicon controlled rectifiers, it is desirable to operate the device at the highest permissible steady state currents without exceeding the thermal limits of the for short duration high current pulses are subject to failure due, for example, to thermal cycling of current pulses of lower amplitude and longer duration against for which there is no need or desire for the circuit to be interrupted. The present invention is directed to providing a fuse which enables devices such as semiconductor devices to be operated at as high a steady state current as the materials of the semiconductor device will permit, yet provide rapid circuit interruption for transient currents of abnormally large amplitude and short time duration which would damage the semiconductor device.

A Reference is now made to FIG. 3 which shows in schematic form the character of the currents which may flow in circuits connected to a device such asa silicon controlled rectifier. The circuits may have a normal operating current of I and include pulse components having amplitudes I and I and 1 respectively with the relative durations indicated. it is desired that fora current flow of amplitude 1 that interruption be prompt and rapid, for example, within several milliseconds. Prior art devices which provided rapid interruption of large amplitude short duration pulses of relative amplitude l would also provide interruption for the current pulses of amplitudes of I, and I,of longer durations and lesser amplitude than I for which it is not desired or required that such interruption be provided. Themanner of response of the fuse of the invention to amplitude versus time profiles such as shown in FIG. 3 will be described and explained in connection with FIG. 4 which showsa graph 30 representing the heat transfer characteristics of a system'such as the fusible elements 21 and the dielectric liquidsystem of thefuse. Along the ordinate of the graph 30 on a logarithmic scale is plotted the thermal power input or heat flux in terms of the watts per square centimeter of surface area 1 of thefusible elements 21. Along the abscissa isplotted the difference in temperature in degrees Centigrade of the liquid 22 and the fusible conductive elements 21.

p the elements 21 is raised heat is transferred from the elements 21 by convection through the liquid 22 and then by surface evaporation at the liquid-vapor interface. As the heat flux is raised to point 31, nucleate boiling is established. As the heat flux is raised above point 31, the power input into the liquid increases very rapidly for a slight increase in temperature difference between liquid and heating elements 21 until a transition point 32 is reached. A still further increase in cur- I rent raising the power input to the liquid, causes vapor film to appear on the elements 21 and such vapor film rapidly encompasses each of the elements. Without a reduction in current flow vapor film boiling becomes complete and the temperature difference between the elements 21 and the liquid 22 required for the flow of thermal powerfrom the elements 21 into the liquid becomes very large as is evident from the graph 30. The

part of graph 30 from abscissa to point 31 is referred to as the interface evaporation region. The part from point 31 to point 32 is referred to as the nucleate boiling region. In the nucleate boiling region the dielectric fluid is at the boiling temperature thereof for the saturation pressure existing in the space 23 of FIG. 1. Preferably, such saturation temperature is maintained close to ambient temperature by augmenting the heat transfer surfaces of the housing 11 by attaching fins l5 thereto. With a large heat transfer surface area the saturation temperature of the liquid is close to the ambient. temperature. The temperature difference region above point 32 of the graph 30 is referred to as vapor film boiling region. The coefficient of heat transfer of the liquid-fusible element system. in the nucleate boiling region is very high in relation to'theheat transfer coefficient for the vapor film boiling region, for example, it is about times greater. The fuse may be designed so that for a current value of 1,, the thermal power inputinto the fuse would be represented by level q, on the graph of FIG. 4. Also, current values I and 1;,

would correspond to levels of q, and q;, of greater thermal power input. The current flow of value I would correspond to a thermalpower input of q which is in excess of the peak thermal power input for nucleate boiling corresponding to point 32. Thermal power input of value q, causes a rapid attainment of the vapor film boiling condition and a rapid rise in temperature of the fusible conducting elements 21- occurs with consequent melting thereof. Accordingly, itisseen that in the device of FIGS. 1 and 2 the plurality of low thermal mass fusible conducting elements are in simple convective and nucleate boiling relationship to a dielectric fluid for normal largeoperating currents with resultant high heat transfer coefficient. However, for very large currents of short duration the fusible elements are in a vapor film boiling relationship to the dielectric liquid with resultant heat transfer coefficient which is substantially lower. Accordingly, heat buildup and temperature rise of the fusible elements is rapid with resultant rapid circuit interruption.

One physical embodiment of the device of FIGS. 1 and 2 to provide a steady-state current carrying capacity of 500 amps could include fusible conductive elements in the form of 40 parallel elements of tin 12 mils (one-thousandth of an inch) in diameter. The liquid could be fluorinated hydrocarbon such as Refrigerant- 2l which is a liquid in the temperature range of 0 to I F and which has a saturation pressure at those temperatures of about 4.5 pounds per square inch absolute and about 55 pounds per square inch absolute, respectively. The fusible conductive elements when subject to a 10 fold increase in current level, that is a current of 5,000 amperes, for a short period of time, would melt in about l millisecond.

Reference is now made to FIGS. 5, 6, and 7 wherein is shown another embodiment of a fuse in accordance with the present invention. The fuse 40 includes a housing 41 having a conductive top wall 42 and a conductive bottom wall 43, a pair of side walls 44 and 45 and a pair of insulating walls 46 and 47 sealed to form a parallelepided enclosure. The top and bottom walls 42 and 43 and the side walls 44 and 45 may be constituted of a material such as copper and the end walls may be constituted of a material such as ceramic, plastic, or metal fitted with insulating bushings to provide electrical insulation for the current leads 48 and 49. A pair of lead-in conductors or electrodes 48 and 49 are provided, extending through end walls 46 and 47, respectively, into the interior of the housing. Electrodes 48 and 49 may be made of a material such as copper which is suitably bonded or sealed to the end walls 46 and 47 and function as terminals for connecting the fuse in the circuit to be protected. The fuse also includes a fusible assembly 50 in the form of an insulating substrate 51, for example, a ceramic wafer on which are deposited the individual fusible, conductive elements 52. While the conductive elements are shown of appreciable thickness, they may be as thin as desired. The conductive elements 52 may be vapor deposited metallic materials of melting temperature suitable for use in fuses, such as zinc or silver. A pair of end conductive members 53 and 54 are provided, one of which is soldered to the ends of the elements 52 at one end of the substrate and the other of which is sol dered to the ends of the elements at the other end of the substrate. The end members 53 and 54 also may be constituted of a material such as zinc or silver. The fusible assembly 50 is located inside the housing and each of the conductors 48 and 49 are conductively connected to end members 53 and 54, respectively, by soldering or by other suitable means. The housing 41 is partially filled with a dielectric liquid 55 which completely submerges the fusible elements 52 in their operating position. The remaining portion 56 of the enclosure may be evacuated so that the space is filled with vapors from the dielectric liquid. Of course, the liquid would not solidify at the ambient temperature in which the fuse would be operated. Also, preferably, sufficient liquid is provided in the'housing to completely submerge the elements 52 regardless of the position v thereof. Also, preferably the liquid has arc quenching device 10 of FIGS. 1 and 2 are designated by reference I numerals corresponding to those used for the device of FIGS. 1 and 2. In the embodiment of FIGS. 8 and 9, the fins 15 have not been included and a porous tubular sheath 61 of insulating material, such as fiberglass, is provided about the elements 21 of the fusible assembly. The porous sheath 61 is cylindrical in form and is mounted between a pair of cylindrical spacer elements 62 and 63. Spacer element 62 is mounted about a conductor l6 and spacer element 63 is mounted about conductor 17. One end of the sheath 61 is clamped to the spacer 62 by means of clamp 64 and similarly the other end of the sheath 61 member is clamped to the spacer 63 by means of clamp 65. Prior to clamping the sheath in place, it is packed to a preselected porosity with a granular arc quenching material 66, such as sand packed therein about the conductive elements 21. In embodiments wherein dielectric liquids are used which do not have sufficient arc quenching properties, the sand provides such properties to shorten the time of response of the fuse without deleteriously affecting the heat transfer phenomena on which the operation of the fuse depends.

While the particular embodiments of this invention has been shown, it will, of course, be understood that the invention is notlimited thereto since modifications both in the arrangement and in the instrumentalities employed may be made. It is contemplated that the appended claims will cover any such modifications as fall within the true spirit and scope of this invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. A current limiting fuse comprising: a housing providing a sealed enclosure; a pair of electrodes, said electrodes supported in insulated relationship in said housing, each of said electrodes extending into said enclosure; at least one fusible conductive element located within said enclosure and connected between said electrodes; a dielectricliquid partially filling said enclosure and completely submerging said at least one fusible conductive element, a first predetermined current flow in said at least one fusible conductive element producing a predetermined thermal power flow into said liquid sufficient to produce nucleate boiling of said liquid but insufficient to produce complete vapor film boiling thereof, a second predetermined current flow in said at least one fusible conductive element producing a second predetermined thermal power flow into said liquid sufficient to produce complete vapor film boiling of said liquid whereby thermal energy accumulates in said at least one fusible conductive element at a rapid rate and raises the temperature thereof at a rapid rate, said at least one fusible conductive element being constituted so that said second predetermined thermal power flow raises the temperature thereof above the melting point thereof; and, a packing of granular arc 7 v r qu enching'ma terial about said at least one fflsible conductive element. 1

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1. A current limiting fuse comprising: a housing providing a sealed enclosure; a pair of electrodes, said electrodes supported in insulated relationship in said housing, each of said electrodes extending into said enclosure; at least one fusible conductive element located within said enclosure and connected between said electrodes; a dielectric liquid partially filling said enclosure and completely submerging said at least one fusible conductive element, a first predetermined current flow in said at least one fusible conductive element producing a predetermined thermal power flow into said liquid sufficient to produce nucleate boiling of said liquid but insufficient to produce complete vapor film boiling thereof, a second predetermined current flow in said at least one fusible conductive element producing a second predetermined thermal power flow into said liquid sufficient to produce complete vapor film boiling of said liquid whereby thermal energy accumulates in said at least one fusible conductive element at a rapid rate and raises the temperature thereof at a rapid rate, said at least one fusible conductive element being constituted so that said second predetermined thermal power flow raises the temperature thereof above the melting point thereof; and, a packing of granular arc quenching material about said at least one fusible conductive element. 